1. Field of the Invention
The present invention relates to an apparatus for calculation of a correlation value concerning a spreading technique to be used such as for radio communications. In particular, it relates to a correlation value calculation apparatus that can take a correlation even when a frequency error range is relatively large.
2. Description of the Related Art
For example, in the form of a CDMA (Code Division Multiple Access) system, a radio communication is performed by a method in which a coded audio data with a bandwidth of 10 kHz for example is spectrum spread to a bandwidth of 1.25 MHz, to have the same band shared by a plurality of communication channels. At a reception end, a correlation is taken with a received spread code, and a reverse spreading process is performed for demodulation and extraction to reproduce a transmitted original data. There is a form of W-CDMA (Wideband-Code Division Multiple Access) system.
As the frequency of a radio signal to be received, there is used a band of several GHz. Therefore, unless the frequency conversion is made with a frequency accuracy of 1 ppm for example, it becomes difficult, as a problem, to take a correlation with a spread code within a received signal to reproduce a data, as a frequency error develops.
Next, there will be shown below how to determine a correlation value Corr(T). A measurement signal S(t) can be obtained by superposing a spread code L(t) on a transmitted data D(t).
The correlation value Corr(T) can be determined by the following Expression 1.
                              Corr          ⁡                      (            T            )                          =                                                                        ∑                t                            ⁢                                                          ⁢                                                S                  ⁡                                      (                    t                    )                                                  ×                                                      R                    *                                    ⁡                                      (                    t                    )                                                                                            2                                    Expression        ⁢                                  ⁢        1            
In this Expression 1, S(t) is a measurement signal (spread wave), R(t) is a reference signal, R*(t) is a complex conjugate of R(t), t is a correlation range, and T is a retrieval range.
In the Expression 1, the retrieval range (T) is calculated, and a highest correlation value is given at a position where a correlation is obtained In this system, however, for a measurement signal with a frequency error, there occurs a case in which the correlation cannot be taken. As an example, there is shown in FIG. 13 a characteristic of “frequency error” vs. “correlation value” at a correlation peak point under the following conditions. The conditions of FIG. 13 are as in a characteristic graph for a data rate of 16 ksps (BPSK), a spread rate by using an orthogonal gold code of 4.096 Mcps (QPSK), a spread ratio of 256, and a correlation range of 6 symbol data. However, the absolute value of the correlation value is changed by the measurement signal.
Next, there is shown in FIG. 14 a “frequency error” vs. “noise/correlation value ratio”, where the “noise/correlation value ratio” is a ratio of a noise by correlation error to a correlation value at a correlation peak value.
As in the graph of FIG. 14, it can be seen that, as the noise is increased with an increase in correlation error, a practical frequency error clearly discriminative of a correlation value resides within approximate 2 kHz or near. In other words, for the data rate of 16 ksps and the correlation range of 6 symbol data, it is to be allowed merely up to a frequency error of approximate 2 kHz or near to determine a point matching with a known data by taking a correlation. On the other hand, in the radio communication which uses a high frequency range of a band of several GHz, it is allowed for a correlation to be taken merely within a frequency error of 1 ppm or near, as a difficulty. There is thus needed an improved analog performance.